1. Field of the Invention
The present invention relates to a semiconductor laser for use as a light source in a digital optical communication system, and a system for and method of performing digital optical communications using such a semiconductor laser.
2. Description of the Related Art
Conventional semiconductor laser devices for digital optical communications widely use a xcex/4 phase-shifted distributed-feedback semiconductor laser. When an optical output signal that has directly been modulated by such a semiconductor laser device is transmitted through an optical fiber, the waveform of the optical output signal is spread by the dispersing effect of the optical fiber, lowering the bit error rate.
FIG. 1 of the accompanying drawings shows an arrangement of a communication system which employs a general optical fiber.
As shown in FIG. 1, electric signal ES1 representing information with 0s and 1s is applied to energize a semiconductor laser 1201 as an electro-optical transducer, which converts electric signal ES1 into optical digital signal OS that is transmitted through optical fiber 1202. Optical digital signal OS that is outputted from optical fiber 1202 is applied to receiver 1203, which converts optical digital signal OS into digital electric signal ES2.
Optical fibers that are most widely used in the 1.55 xcexcm band in optical communication systems have a dispersing effect such that optical signals having higher frequencies travel through the optical fibers at higher rates and optical signals having lower frequencies travel through the optical fibers at lower rates. Direct-modulation semiconductor lasers for use as an electro-optical transducer have general characteristics such that the frequency of an optical signal produced thereby is lowered when the optical intensity thereof varies. Therefore, the optical signal that has been generated by the direct-modulation semiconductor laser and transmitted through the optical fiber has its waveform degraded.
Semiconductor lasers are roughly classified into two types, i.e., a semiconductor laser combined with an external modulator as disclosed in Japanese laid-open patent publication No. 07-106691 and a direct-modulation semiconductor laser as disclosed in Japanese laid-open patent publication No. 04-156120.
The former semiconductor laser that is combined with the external modulator is energized to produce an optical signal with a constant output level, which is modulated by the external modulator. Therefore, any frequency variation of the output optical signal is very small, and the output optical signal can be transmitted over a distance of several hundreds km. However, the overall assembly is complex and relatively costly.
The latter direct-modulation semiconductor laser suffers very large optical signal frequency variations as the optical signal intensity varies. It has been reported that after an optical signal produced by the direct-modulation semiconductor laser has been transmitted over a distance of 100 km, it suffers a high power penalty of several decibels. The power penalty represents a degradation of the reception sensitivity which is estimated as a substantial loss of the received power of the optical signal. Consequently, the direct-modulation semiconductor laser is not suitable for use as a light source for producing optical signals that are to be transmitted over long distances.
It is therefore an object of the present invention to provide a digital optical communication method for transmitting an optical signal over a distance of 100 km with a power penalty reduced to 1 decibel or less or a negative value, an inexpensive semiconductor laser suitable for use in such a digital optical communication method, and a digital optical communication system which employs such a semiconductor laser.
According to the present invention, there is provided a method of performing digital optical communications to transmit an optical signal through an optical fiber, comprising the step of shaping the waveform of the optical signal to be transmitted through the optical fiber to increase the frequency thereof before the waveform is stabilized when the optical signal starts increasing in level at the time the optical signal is applied to the optical fiber.
According to the present invention, there is also provided a semiconductor laser comprising a diffraction grating for effecting distribution feedback, the diffraction grating having a normalized coupling coefficient xcexaL of at least 2.0, the diffraction grating having a phase shift region disposed therein for achieving a phase shift of at most xcex/4, and an active layer having a gain which is saturated as a carrier concentration in the active layer increases.
The semiconductor laser may further comprise a resonator, the phase shift region being disposed nearly centrally in the resonator.
The active layer may have a multiple quantum well structure having growth surface irregularities.
The active layer may have a multiple quantum well structure composed of two stage potential quantum wells.
The active layer may have a multiple quantum well structure including a non-radiative carrier recombination layer.
The active layer may have a multiple quantum well structure which is progressively thicker toward the center of the semiconductor laser in the axial direction of the resonator.
According to the present invention, a digital optical communication system has the above semiconductor laser used as a communication light source.
The above and other objects, features, and advantages of the present invention will become apparent from the following description with reference to the accompanying drawings which illustrate examples of the present invention.